Brake lining materials and articles made therefrom



March 21, 1967 c. H. SUMP ETAL I 3,310,387

BRAKE LINING MATERIALS AND ARTICLES MADE THEREFROM Original Filed Aug.24, 1960 INVENTOR.

CORD H. .SUMP

BY SHERWOOD M665 United States Patent 6 Claims. (Cl. 29-1825) Thepresent application is a continuation of our copending application Ser.No. 51,709, filed Aug. 24, 1960, and now abandoned, which is in turn acontinuation-inpart of our copending application, Ser. No. 671,992,filed July 15, 1957, and now abandoned.

The present invention relates to brake lining and other frictionalmaterials, and more particularly to such materials formed of a compositecomprising randomly oriented bonded metal fibers and ceramic materials.It fur ther relates to articles such as brake shoes fabricated from suchcomposites. In the compositions produced in accordance with ourinvention, at substantially all points of contact between metal fibers,autogenous, or metal-tometal bonds are produced, and in a major portionof the material, where contact permits, metal-to-ceramic bonds are alsopresent. The fibers extend in all directions within the composition andform a random three dimensional array. In some of our compositions thereare essentially only two ingredients, namely the metal fibers and asingle ceramic constituent, in which case the ceramic is bonded directlyto the metal whereas in others of our compositions a third ingredient,primarily another ceremic type material, is added to assist and takepart in the various bond formations requisite of the instant materials.Additionally, in those instances where required, a lubricant materialmay be incorporated within our compositions.

Along with the development of high speed transportation, particularlyaircraft, a need has resulted for brake lining materials which arecapable of withstanding high temperature operation while maintainingtheir physical structure and functional abilities. true for jet aircraftbraking systems for the landing speeds involved are far greater thananything hithertofore known with consequent high degree of brakeburn-out, destruction and system failure. We have developed new compositions which illustrate remarkably good thermal stability accompaniedby excellent braking and frictional properties which are particularlygood for aircraft use.

It is, therefore, an object of the instant invention to provide improvedbrake lining materials capable of sustained operation at hightemperature.

Another object of our invention is to provide brake shoes and the likemade from such compositions.

Another object of the instant-invention is to provide brake liningmaterials having excellent thermal and mechanical stability which areformed of refractory or semirefractory metal fibers andrefractorycera-mic materials, such metal'fibers being characterized byhaving metal-to-metal (i.e., autogenous) bonds at their points ofjunction and such material being further characterized This isparticularly 3,310,387 Patented Mar. 21, 1967 2 by havingmetal-to-ceramicbonds', and wherein both types of bonds extend in alldirections to form a three-dimensional network. a

A further object of the instant invention is to provide brake liningmaterials having ,a randomly oriented bonded fiber metal skeletalstructure.

Otherobjects, features and advantages of the instant invention will beobvious to those skilled in this particular art from the followingdetailed disclosure hereof and the accompanying drawings.

In producing these novel frictional materials We have used variousrefractory and semi-refractory metals selected from Groups IV throughVII of the periodic table. Metals such as molybdenum, vanadium, nickel,titanium, iron, tantalum, zirconium, niobium, thorium, cobalt, chromiumand tungsten, either singly or in combination, may all be used to formthe fiber metal skeleton or framework of the instant materials.Furthermore, useful fibers may also be selected from alloys of suchmetals having melting points above 2000 F. as for example,those composedof titanium and 30% molybdenum, or 50% titanium and 50% vanadium, orstainless steel. 0 At the onset it should be understood that by the termmetal fiber is meant elongated metal bodies having discrete,limitational lengths, the actual dimensions thereof being determined inpart by factors such as the sizes of the formation vessel and of the endproduct itself. In the terminology of the fabric fiber industry staple,or fairly short fibers, as compared with very long filaments, are usedin the practice of the instant invention.

Even more specifically, as employed in the present specification andclaims, by the term fiber is meant an elongated metallic body having along dimension substantially greater than its mean dimension in crosssection. As a general rule for applicability herein, a fiber should havea length at least about ten times its mean dimension in cross section.Such latter term is related to the shape of the fiber in cross sectionand refers to the diameter of the cross section in the case of acircular fiber, or in the case of a rectangular sectioned member denotesone-half the sum-of the short side and the long side of the rectangle.The ratio of fiber length to its mean dimension in cross section mayvary from ten to one to 500 to one.

We have achieved particularly good results in forming the present brakelining materials by employing metal fibers having a diameter of 0.00025inch to 0.010 inch, and lengths varying from about 0.02 inch to twoinches. In order to achieve a randomly oriented fiber metal skeleton,care should be taken to employ fibers having lengths of two inches orless. Beyond such maximum length value, one enters into the'area of themetal wools, which consist of rather long length filaments and which.are not useful in forming the desired random metal fiber arrays. Themetal wool filaments cannot be randomly felted.

In the drawings:'

FIGURE 1 illustrates a' friction element for a disc brake produced inaccordance with the instant invention;

FIGURE 2 illustrates a microscopic view of the instant frictionalmaterial structure, said view being enlarged approxiamtely 200diameters; and

FIGURE 3 a typical brake shoe produced in accordance with the instantinvention.

As illustrated in FIGURE 1, the friction element has a rubbing surface11 which presents a discrete, staple fiber appearance. A hole 12, isbored through the axis of the element to permit mounting for operationuse.

In FIGURE 2, the actual structure of the instant materials is best seen.This is composed of a metal fiber skeleton 13, for example, ofmolybdenum, such skeleton comprising relatively short length metalfibers of the size above noted, autogenously bonded at points ofcontact, and further, being bonded to ceramic and carbon materialseither directly, or through an intermediate bonding agent. In thedrawing the ceramic particles are indicated by the numeral 14, and thecarbon granules by 15. The numeral 16 represents the intermediatebonding agent, as for example, ferrosilicon. This intermediate bondingagent effectively bonds the ceramic materials to the metallic skeleton.

In FIGURE 3 is seen a brake shoe produced in accordance with the instantinvention. This has a support member 17, having rivet holes 18 thereinfor mounting purposes. A steel plate 19, or the like, is superimposedupon the support member, and located upon such plate is the skeletalfiber friction element 20.

It is known in this art that the so-called cermets may be used asfrictional materials, but in most cases this term deals primarily withmetal powder-ceramic combinations which as a practical matter do nothave the mechanical stability illustrated by the compositions of theinstant in-' vention. To the best of our knowledge nowhere does theprior art indicate the usage of autogenously bonded metal fibers,particularly in a continuous phase, and the superior frictionalmaterials so produced. Although the products of the instant inventionmay -be categorized as cermets generally for they do consist of ametal-ceramic combination, in view of their superior properties, theyshould be considered as a special material thereunder.

We have found that the use of autogenously bonded metal fibers greatlyimproves the frictional and brake lining characteristics of a cermet.Among other things, there is a change in the failure characteristics ofsuch bodies when subjected to compressional stresses. Instead of thetypical brittle fracture found Within the usual, i.e., powder-metalcermet material, in the instant invention there is an apparent ductileyielding upon compression loading. Because of the presence of thefibers, an incomplete microfracture occurs rather than the completeseparation of materials. As a result of the autogenous bonds incombination with the fibrous structure, the entire mass tends to hangtogether even despite severe stresses. Along this line also, thepresence of a ductile metallic phase in metal fiber form reduces localstress concentrations and in this manner further inhibits thepropagation of cracks within the fiber metal-ceramic body. In varioustests performed to explore the effects of thermal shock on the instantcompositions, we found that the metal fibers promote a manyfoldresistance to such shock. This improvement apparently stems from atleast two sources, viz., first, crack propagation is arrested by thepresence of the ductile fibers, and second, the continuous phase fibersform a heat conductive path through the ceramic material and thus reducethe thermal gradient from the surface to the core of the structure. Withsuch reduction in the thermal gradient thermally incurred stresses inthe body are likewise reduced so it can necessarily better withstand thetemperature changes to which it is subjected. Still another contributionof metal fibers to tional purposes.

such improved cermet materials is the much greater oxidation resistanceof the latter. We observed that for equivalent compositions of metalpowder cermet and molybdenum fiber cermet that the rate of oxidationattack in an identical high temperature environment is much slower forthe bonded fiber material.

The metal fiber cermets illustrate improved toughness and resiliencywhen compared with the powder metal. Such characteristics areparticularly evident when the duetility of the fiber is retained, as forexample, when nickel fibers are used the resultant material illustratesextreme resistance to fragmentary separation under compressiveloadingeach ceramic fragment is held attached by the fibrous metalstructure. Our materials are caused to disintegrate only after extremedifficulty. Even in these instances where an entire ceramic phase hasbeen broken by mechanical damage, the fiber structure is still retained.The resiliency factors are, of course, of the utmost importance when theinstant materials are utilized for fric- There are secondary effectshere, such as improved damping properties which are related to chatterand other mechanical difficulties currently encountered in frictionmaterials.

In brake lining materials made in accordance with our inventionmicrostructural changes are possible in the placement of the ceramicconstituent whereas in powder cermets, because of their inherentgeometry, such placement changes are not possible.

In order to obtain the superior properties of the instant materials, itis essential that the metal fibers be welded to each other at theirpoints of contact, i.e., autogenous bonds must be formed substantiallythroughout the entire brake lining mass. Such a weld, or bond is quitesimilar to the bond developed when metal powders are sintered and may bedefined as a sintered bond. Once the skeletal network has been formed,the ceramicphase may be introduced by such techniques as infiltration orslip casting; however, in most instances, the metal fiber skeleton isformed simultaneously with the formation of the ceramic components.

Metal fiber skeletons, useful with the present invention, may be formedby first felting the constituent fibers through air into a mold orsuitable container having the form of the final composite in crosssection. In this condition the fibers form a weakly coherent mass whichis transferred to a furnace provided with a non-oxidizing atmosphere.The mass is then heated to its sintering temperature, i.e.,approximately to of the melting point of the metalto result in theformation of strong metallic bonds at points of fiber content. Theceramic phase is then introduced into the fiber metal pores by suchtechniques as passing a ceramic slurry therethrough.

Another important requirement of the instant composition is thenecessity for a strong bond between the ceramic and the metal. If suchmetallic-ceramic bond is not present, failure oftentimes occurs at theinterface therebetween primarily because such interface acts as thestress concentration area. This type of bond is also important in orderthat the ceramic may take full advantage of the conductive properties ofthe continuous metallic phase; without the intimacy of contact there ispoor heat transfer through the material mass.

In order that our invention may be fully understood, the followingspecific examples of compositions made in accordance therewith arepresented. These mixtures were used to prepare one-quarter inch thickbuttons of two inch diameter for a disc brake, as shown in FIGURE 1. Forother size buttons, of course, the fiber length could be varied.

Example I Kinked molybdenum fiber 0.003 inch to 0.008 inch diameter,

Ferrosilicon containing 15% by weight iron, mesh 10% by vol. or 4.1%:

by weight.

Example IContinued Carbon, calcined refined Texas Petroleum Coke, 72+100 mesh by vol. or 3.5%

. by weight.

The molybdenum fiber as received was bundled and cut by shearing to therequired length. The cut fiber was tumbled in a drum to produce kinkingbut the formation of tight fiber clumps was avoided. At this point, nobonds were present between the constituents and the structure supporteditself by mere mechanical interlocking. The preformed compact thusformed was placed in a graphite die and hot pressed at 2500 F. and 6000p.s.i. pressure. The simultaneous application of heat and pressure inthe process of hot pressing insures the establishment of metal-ceramicbonds.

The hot pressing is performed by confining the preformed article withina die, heating it to a temperature at which the ceramic is surfaceactive so that bonding can occur and then applying pressure at thattemperature. Temperatures between 2500 F. and 3200 F. and pressures of6000 p.s.i. have been successfully employed.

The properties of the end product depend upon the time permitted toraise the mass to this temperature and the interval during which it isheld at such elevated temperature. The hot pressed compact may bemachined, ground or lapped at the expiration of such operation.

Example II Titanium Wool, 0.003 inch to 0.010 inch diameter, 0.5 inchlengths 50% by vol. or 61.4%

by Weight. Green silicon carbide, +35

+48 mesh 30% by vol. or 26.0%

by weight. Silicon, 100 mesh 10% by vol. or 6.5%

by weight. Carbon, calcined refined Texas Petroleum Coke, 72 +100 mesh10% by vol. or 6.1%

by weight.

In this mixture the procedure for forming is the same as that forExample I except that hot pressing is performed at 2650 F.

We found that there are practically no limitations as to the type ofmanufacturing process which yields the metal fibers useful in theinstant invention, and either cut wire or cut wire wool may be used. Inmany cases our experimental linings utilized fibers cut by merelybundling masses of metal wire and/or wool and then shearing to thedesired length. There are however, certain characteristics orrequirements of the utilizable fibers which should be obtained in orderto have the superior end products hereof, to wit:

(1) The fibers should preferably have a diameter of from 0.0003 to 0.010inch.

(2) The fibers must be of a feltable length to deposit as a randomlyoriented three-dimensional array. The critical length limitations havebeen set out above.

(3) Optimum properties are achieved when the fibers are kinked. By thisis meant that the directional orientation of each fiber is changed atleast three times along its longitudinal axis. Such kinking improves thefrictional properties by contributing mechanical interlocking of thefibers themselves and the fibers to the non-metallic additives. Inservice, the kinked fiber retains the nonmetallics of the lining moreeffectively than a straight length fiber. Additionally, improveduniformity of the friction blend results from this kinked condition.

(4) It is apparently essential that the ductility of the metal fiber beretained during the manufacture of the lining. By ductility is meant theability of individual fibers to be permanently bent without breaking.

Many ceramic materials may be utilized with the bonded metal fibers toproduce the instant frictional materials. Certain ceramiccharacteristics should also be provided, as for example:

(1) The ceramic must be capable of being bonded to the metal fiberduring the manufacturing process, either directly or through the use ofan intermediate bonding agent as cited previously.

(2) Softening temperature should be as high as possible, subject ofcourse to the bondability requirement. For example, mullite, (3Al O.2SiO has a higher softening temperature than silica alone and is foundto produce a more satisfactory brake lining. On the other hand, A1 0 hasa higher softening temperature than mixtures thereof with silica butthere is greater difficulty in establishing satisfactory bonding to themetal fibers.

(3) Coarser particles, i.e., in the range of from 35 to +48 mesh arepreferred over the finer fractions.

In some of the instant brake lining compositions it is desirable to addlubricating agents to the fiber metalceramic mass. For examp e, we haveused lead, molybdenum, disulfide, and various forms of carbon for thispurpose. As above indicated, in many cases, carbon is added to the rawmaterial charge. Crushed calcined petroleum pitch coke of from 72 tomesh was used extensively, and we found also that graphite of the sameparticle size was quite useful.

Mixtures of alumina and silica were used in practically all proportionsto study their friction characteristics and in many cases such mullitecompositions having particle sizes of from 35 to +48 mesh were used.

We found that the best brake lining materials have the non-metallicconstituents composed of silicon carbide in conjunction with silicon orferrosilicon, and additionally, in many instances, a form of carbon.Preferably, the particle size of the crushed carbide is from 35 to +48mesh. For our purposes, crushed green, electric furnace silicon carbidewas used almost exclusively, it of course being understood that othertypes of like material from other sources may be used. Silicon orferrosilicon of approximately 200 mesh yields excellent end products.Such silicon and/ or ferrosilicon not only bonds the carbide particlesinto essentially a continuous phase, but additionally bonds the siliconcarbide, and, Where used, carbon, to the metal fibers.

It should be understood that the bonding of silicon carbide with siliconor ferrosilicon does not result in an equilibrium mixture. For thisreason the heating time and the temperature employed in the manufactureof the instant materials are selected in order to retain as much of theoriginal carbide and metal fiber as is practical. Heterogeneity isdesirable, rather than a fully fused ceramic constituent.

The use of the silicon or ferrosilicon stems from the inability, in somecases, of the metal fiber to properly surround the ceramic particles andhold them in place. If, for example, molybdenum fibers are used toreinforce silicon carbide, the silicon or ferrosilic on additive isnecessary. Since silicon forms compounds with both carbon andmolybdenum, it provides a means of bonding these materials to withstandsevere braking service. Examination of microstructures of Examples I andII above shows that intermediate phases exist between metal fiber andsilicon (or ferrosilicon). The silicon was in intimate contact with thesilicon carbide granules, having wet them, and thus assists in theirfirm support in the lining matrix. We also found that metal powders orfilms may be employed as additional bonds to obtain intimate contactbetween the metal and ceramic granules.

A few test results of the instant brake lining materials are as follows:

3 3. A friction material consisting essentially of: a substantiallyautogenously bonded, randomly oriented, fiber Friction Material Type ofTest Test Results By volume, 50% molybdenum fiber, 30% SiC, Fe-Si, 10%carbon, as cited under Example I. 7

full size aircraft disc brake.

iug performance.

Resin Base Standard Control Lining. This lining was tested in con--junction with a titanium alloy mating member.

full size aircraft disc brake.

ing performance.

size aircraft disc brake.

Inertia type dynamometer, for use with 50 or more steps required todestruction for qualify- Inertia type dynamometer, for use with 50 ormore stops required to destruction for qualify- Drag type dynamometer'test capacity 108,000 ft.-lb./min./in. speed 1,840 f.p.m.

Inertia type dynamometer for use with full 50 or more stops required todestruction. Tested using titanium alloy mating member;

A total energy of 4,560,000 foot pounds per square inch of liningsurface was dissipated in 94 stops with 0.00007 inch to 0.00006 inch perstop lining Wear. Linings were still in serviceable condition aftertesting. Coefficient of friction ranged from 0.11 to 0.15. Temperaturesfrom 1,800 to 2,000 F. were estimated during these tests. This frictionmaterial was found to be compatible with steel and titanium alloy matingmembers.

A total energy of 103,000 foot pounds per square inch of lining surfacewas dissipated in 5 stops. This friction material was found to becompatible with steel mating members. Average lining wear was 0.0005inch per stop, friction coeflicient 0.25.

A minute drag test at an average energy absorption rate of 04,100 footpounds per square inch per minute was completed. The friction materialwas still serviceable after the test. Wear rate was less than 0.00002inch per minute. Temperature during test was 1,740-1,780 F. Coelllcientof friction was 0.19 to 0.22.

A total energy of 135,200 foot pounds per square inch of lining surfacewas dissipated in three stops. Linings were destroyed at this point.Lining wear in this test was 0.180 inch per stop.

Under conventional conditions with steel mating members,

' these linings will last 50 stops. Lining wear being about 0.002

inch per stop.

All fiber metal compositions shown in the preceding table were preparedby the blending operation described above and were then hot pressedusing carbon punches and graphite dies. The molybdenum fibercompositions were hot pressed at 2500 F. while that containing thetitanium fibers was hot pressed at 2650 F.

The metal fibers should be present in amounts ranging from to 80% byvolume, with the preferred amount being 50%, the remainder being theother constituents. The primary ceramic constituent ranges from 20% to80% by volume also. i

It will be understood that modifications and variations may be effectedwithout departing from the spirit or scope of the novel concepts of ourinvention.

We claim as our invention:

1. A friction material consisting essentially of: a substantiallyautogenously bonded, randomly oriented, fiber metal support, each fiberof said support being kinked and having a length ranging from 0.002 to2.0 inches, a mean dimension in cross section of from 0.00025 to 0.01inch, and a length to mean dimension in cross section ratio of at least10 to 1, said fiber metal being selected from the group consisting ofmolybdenum, vanadium, nickel, titanium, iron, tantalum, zirconium,niobium, thorium, cobalt, chromium and tungsten, and alloys and mixturesthereof, said fiber metal support comprising 20% to 80% by volume ofsaid friction material, and a refractory ceramic comprising the balancethereof, said refractory ceramic being bonded to said fiber metalsupport by a material selected from the group consisting of silicon andferrosilicon.

2. A friction material consisting essentially of: a substantiallyantogenously bonded randomly oriented, fiber metal support, each fiberof said support being kinked and having a length ranging from 0.002 to2.0 inches, a mean dimension in cross section of from 0.00025 to 0.01inch, and a length to mean dimension in cross section ratio of atleast10 to 1, said fiber metal being selected from the group consisting ofmolybdenum, vanadium, nickel, titanium, iron, tantalum, zirconium,niobium, thorium, cobalt, chromium and tungsten, and alloys and mixturesthereof, said fiber metal support comprising 20% to 80% by volume ofsaid friction material; a refractory ceramic bonded to said fiber metalsupport by a material selected from the group consisting of silicon andferrosilicon, and

metal support, each fiber of said support being kinked and having alength ranging from 0.002 to 2.0 inches, 21 mean dimension in crosssection of from 0.00025 to 0.01 inch, and a length to mean dimension incross section ratio of at least 10 to 1, said fiber metal being selectedfrom the group consisting of molybdenum, vanadium nickel, titanium,iron, tantalum, zirconium, niobium, thorium, cobalt, chromium andtungsten, and alloys and mixtures thereof, said fiber metal supportcomprising from 20% to 80% by volume of said friction material, thebalance consisting of a refractory ceramic, said refractory ceramicbeing directly bonded to said fiber metal support.

4. A friction material consisting essentially of: a substantiallyautogenously bonded randomly oriented fiber metal support, each fiber ofsaid support being kinked and having a length ranging from 0.002 to2.0'inches, a 40 mean dimension in cross section of from'0.00025 to 0.01inch, and a length to mean dimension in cross section ratio of at least10 to 1, said fiber metal being selected from the group consisting ofmolybdenum, vanadium, nickel, titanium, iron, tantalum, zirconium,niobium, thorium, 4 cobalt, chromium, and tungsten, and alloys andmixtures thereof, said fiber metal support comprising from 20% to 80% byvolume of said friction material, the balance consisting of a refractoryceramic, said refractory ceramic being directly bonded to said fibermetal support, and a lubricating agent enmeshed in said fibermetal-ceramic support.

5. A fiber metal ceramic support as set forth in claim 4 wherein saidlubricating agent is carbon.

6. A fiber metal ceramic support as set forth in claim 4 wherein saidlubricating agent is molybdenum disulphide.

References Cited by the Examiner UNITED STATES PATENTS 2,898,216 8/1959Bray et al. 29--182.5 2,903,787 9/1959 Brennan.

3,114,197 12/1963 Du Bois et al. 3,127,668 4/1964 Troy 29l82 3,153,27910/1964 Chessin. FOREIGN PATENTS 733,061 7/1955 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

7 L. DEWAYNE RUTLEDGE, Examiner.

R. L. GRUDZIECKI, Assistant Examiner.

1. A FRICTION MATERIAL CONSISTING ESSENTIALLY OF: A SUBSTANTIALLYAUTOGENOUSLY BONDED, RADOMLY ORIENTED, FIBER METAL SUPPORT, EACH FIBEROF SAID SUPPORT BEING KINKED AND HAVING A LENGTH RANGING FROM 0.002 TO2.0 INCHES, A MEAN DIMENSION IN CROSS SECTION OF FROM 0.00025 TO 0.01INCH, AND A LENGTH TO MEAN DIMENSION IN CROSS SECTION RATIO OF AT LEAST10 TO 1, SAID FIBER METAL BEING SELECTED FROM THE GROUP CONSISTING OFMOLYBDENUM, VANADIUM, NICKEL, TITANIUM, IRON, TANTALUM, ZIRCONIUM,NIOBIUM, THORIUM, COBALT, CHROMIUM AND TUNGSTEN, AND ALLOYS AND MIXTURESTHEREOF, SAID FIBER METAL SUPPORT COMPRISING 20% BY 80% BY VOLUME OFSAID FRICTION MATERIAL, AND A REFRACTORY CERAMIC COMPRISING THE BALANCETHEREOF, SAID REFRACTORY CERAMIC BEING BONDED TO SAID FIBER METALSUPPORT BY A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON ANDFERROSILICON.